1. Field of the Invention
The present invention relates, in general, to a random access method and apparatus based on analog network coding for a two-way relay channel and, more particularly, to technology for combining analog network coding with random access, thus guaranteeing high channel usage efficiency upon utilizing the combined technology for a system that uses a two-way relay channel.
2. Description of the Related Art
A satellite network, which is a representative two-way relay channel network, is advantageous in that it can be shared by terminal stations located across a large area, but it has the limitation of high cost for the use of the network. Therefore, in a satellite network, high-efficiency communication techniques optimized for respective cases where the amount of data to be transmitted is large or small have been used.
Generally, in a satellite network, demand assign techniques and random access techniques are used in parallel with each other. Since this random access technique has very low system throughput, its use is limited to the case when the amount of transmission traffic is not large in the satellite network. In particular, it is known that 1970's slotted ALOHA, one random access technique, has an efficiency of 37% and the highest system throughput, and is not used when the amount of traffic is large, as in the case of data traffic of application programs. Such a technique is limited to use only when the amount of traffic is small, as in the case of network control data or system management messages. In contrast, the demand assign technique is used when the amount of traffic is large, and Demand Assigned Multiple Access (DAMA) is chiefly used as the demand assign technique. However, DAMA is problematic in that, when an amount of traffic less than demand-assigned traffic is generated, an amount of network resources corresponding to the difference between actual traffic and basic demand-assigned traffic is wasted.
Further, the random access technique is characterized in that it copes better than the demand assign technique with traffic having intermittent characteristics, but it is problematic in that its use in a satellite network is difficult due to low system throughput. Further, conventional schemes for improving the system throughput of the random access technique have been presented, but the improvement level thereof is not sufficient, and the increase in the efficiency of channel use is limited.
Meanwhile, a satellite communication system has been used to separate an uplink frequency to a satellite from a downlink frequency so as to avoid channel interference. Accordingly, the satellite communication system has, as a basic channel, a two-way relay channel, which utilizes a satellite relay. Further, in a recent relay system, an Analog Network Coding (ANC) technique for increasing throughput up to double that of an existing scheme has been proposed. ANC simultaneously transmits uplink and downlink signals in a single frequency band and a single time slot, and the simultaneously transmitted uplink and downlink signals overlap each other and are then received. The core of the above-described ANC may be regarded as interference cancellation (IC) technology for allowing a certain terminal to recover a received signal by removing a signal transmitted by the terminal from an overlapping signal. Such IC technology may be used independent of modulation technique or channel coding.
It is predicted that such ANC may greatly improve throughput when it is applied to satellite random access. However, when ANC is applied to random access, a problem may arise in that it is impossible to utilize ANC due to unintentional packet collisions. Further, there is a problem in that a physical layer cannot estimate precise channel values for the channel through which each packet is transmitted, so that the resultant error entirely acts as an interference factor, thus making it impossible to recover packets even if interference is cancelled.
Meanwhile, Korean Patent No. 10-0414510 entitled “Temporary Frame Identification for ARQ in a Reservation-Slotted-ALOHA Type of Protocol” presents a method of implementing an automatic repeat request (ARQ) in a mobile packet communications system using a reservation slotted-ALOHA protocol in which data frames, each having multiple data blocks, are exchanged between a base station and mobile stations, the method including the step of assigning, to each data frame transmitted to a mobile station, a Temporary Frame Identity (TFI), wherein the assigned TFI is unique among other TFIs assigned to data frames concurrently transmitted to other mobile stations, and the step of including the assigned TFI in all data blocks in the data frame to which the TFI is assigned.
The preceding technology is a technology using a Reservation ALOHA (R-ALOHA) technique in which a single terminal reserves and utilizes a single slot. Such R-ALOHA is configured to, when the transmission of a packet succeeds, reserve the corresponding slot, maintain the reservation of the slot by continuously transmitting packets in the slot, and terminate the reservation of the slot by inserting an end-of-file flag into the last transmitted packet upon terminating reservation. When channel reservation for ANC is performed using such an R-ALOHA technique, a problem arises in that information inside a packet cannot be determined by terminals other than the two terminals that desire to exchange information using ANC (hereinafter referred to an “ANC pair”), due to the transmission of overlapping signals. As a result, a problem arises in that other terminals cannot determine whether the corresponding slot is a slot in which communication based on ANC is performed or a slot in which packets are colliding with each other.